Add Google Benchmark and workflow

[retrage]

Summary

• expose run_simulation in main.cpp for reuse
• add bench.cpp with Google Benchmark
• support new benchmark target in Makefile
• document how to run the benchmark
• guide agents to run benchmarks when needed
• create benchmark.yml workflow to upload results

Testing

• make
• make bench
• pytest -q
• ./bench --benchmark_out=benchmark.json --benchmark_out_format=json

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https://chatgpt.com/codex/tasks/task_e_6854232fc4c083209d916e0b53b69acb